PROJECT SUMMARY Head and neck squamous cell carcinoma (HNSCC) remains a lethal disease despite concerted efforts to improve its diagnosis and treatment. Although smoking, drinking, and HPV infection are closely linked to HNSCC, emerging evidence also suggests that changes in the human microbiome are associated with HNSCC. However, little is known about how the altered microbiota affect HNSCC pathogenesis or its treatment. Currently, most studies linking the microbiome to cancer have focused on gut microbiota, which either contributes locally to colorectal cancer pathogenesis, or more broadly, modulates metabolism and immunity systemically. The oral cavity, which is more directly relevant to the local microenvironment in HNSCC, harbors one of the most complex and diverse microbiomes of all human anatomical sub-sites. However, how either the gut or oral microbiotas shape the local and systemic microenvironment in HNSCC pathogenesis and treatment are completely unknown. Our long-term goal is to determine the functional role(s) and mechanisms of action of the microbiome in HNSCC and to translate these findings into novel preventative and therapeutic strategies. Our central hypothesis is that dysbiosis arising during HNSCC pathogenesis acts through Toll-like Receptor 2 signaling pathways to accelerate tumor development and compromise response to therapies directed at the PI3 kinase pathway, a key driver of HNSCC. This hypothesis has been formulated on the basis of publications and preliminary data produced in the applicants' laboratories and will be tested by pursuing two specific aims that will: 1) Test the hypothesis that TLR2 signaling mediates microbiome-related HNSCC pathogenesis and 2) Test the hypothesis that dysbiosis-stimulated TLR2 signaling compromises anti-PI3K therapy. Our approach is innovative because it represents a departure from the status quo by utilizing unique experimental mouse models and state-of-art technologies to move beyond observational human studies to delineate the molecular, cellular, and immunological mechanisms induced by the microbiome in HNSCC pathogenesis. The proposed research is significant because it is expected to advance and expand understanding of how the microbiome as a whole, as well as specific microbial species, impacts host immunity in HNSCC development and immunotherapy. Ultimately, such knowledge has the potential to be developed into effective therapies for HNSCC patients, a pressing need given the significant incidence and poor prognosis of this disease.